The present invention relates to a catalyst for exhaust gas purification and a method for exhaust gas purification using the catalyst, particularly to a catalyst for exhaust gas purification which can be suitably used in automobile internal combustion engines and a method for exhaust gas purification using the catalyst. More particularly, the present invention relates to a catalyst for exhaust gas purification which can preferably convert unburnt hydrocarbons (HC) which are present in a large amount in the exhaust gas of gasoline engine automobile at the engine start (i.e. cold start), in the presence of secondary air and, after the engine start, can convert hydrocarbons, carbon monoxide (CO) and nitrogen oxide (NO.sub.x) at a high efficiency stably for a long period of time, and a method for exhaust gas purification using the catalyst.
As regulation for automobile exhaust gas emissions has become stricter, attention is being paid to a technique of arranging a catalyst in the vicinity of engine manifold to improve the catalyst's warm-up property or a technique of using an electrically heated catalyst (EHC) to give rise to quick temperature increase to (1) heat a catalyst supported on the EHC or a catalyst arranged downstream of the EHC, for example, a main catalyst and thereby (2) convert unburnt hydrocarbons which are present in a large amount in the exhaust gas from a gasoline engine of an automobile at the engine start.
There was also proposed a technique of, in addition to the above-mentioned improvement of catalyst's warm-up property at engine start, introducing secondary air to make the exhaust gas leaner than the stoichiometric air-fuel ratio (air excess ratio .lambda.=1) when the exhaust gas is at a fuel-rich side and conduct improved purification of exhaust gas.
An example of the catalysts used in the above techniques for exhaust gas purification is disclosed in Japanese Patent Publication No. 38892/1991. It is a three-way catalyst comprising
a catalyst carrier,
a catalyst layer containing at least either of Pt and Rh as the catalyst component, provided on the carrier, and
an alumina layer containing 50-95% of CeO.sub.2 as an oxygen storability-imparting agent and a small amount of Pd, provided on the catalyst layer.
Catalysts each supported on an electrically heated catalyst (EHC) are disclosed as catalysts for exhaust gas purification, in Japanese Utility Model Application Kokai (Laid-open) No. 67609/1988, Japanese Patent Application Kohyo No. 500911/1991 and Japanese Patent Application Kokai (Laid-open) No. 72953/1991.
Further in Japanese Patent Application Kokai (Laid-open) No. 56247/1990, there is disclosed a three-way catalyst which shows a high conversion efficiency particularly for hydrocarbons in an exhaust gas of stoichiometric to fuel-rich air-fuel ratio at cold start. This catalyst for exhaust gas purification comprises
a carrier,
a first catalyst layer composed mainly of zeolite, provided on the carrier, and
a second catalyst layer provided on the first catalyst layer, having an oxidizing and reducing activity and comprising a noble metal (e.g. Pt, Pd and Rh) supported on a coating layer of Al.sub.2 O.sub.3 or the like.
Further in Japanese Patent Application Kokai (Laid-open) No. 84635/1988, there is disclosed a three-way catalyst wherein the kinds and amounts of supported catalyst metals are different at the exhaust gas inlet and the exhaust gas outlet. It is a catalyst for exhaust gas purification which is intended for improved purification of an exhaust gas generated, for example, at cold start (low temperature) and which comprises
a monolith carrier,
Pd supported on the gas inlet portion of the carrier over the 1/10 to 2/5 length of the total carrier length, and
Pt supported on the gas outlet portion of the carrier over the 3/5 to 9/10 length of the total carrier length. In this catalyst, Rh, which is effective for conversion of NO.sub.x, is supported on the entire catalyst layer. That is, Pd and Rh are supported on the upstream portion, and Pt and Rh are supported on the downstream portion.
In Japanese Patent Application Kokai (Laid-open) No. 101813/1991, there is disclosed a similar three-way catalyst which is a catalytic converter having an improved light-off performance at low temperatures. It is a catalytic converter comprising a plurality of catalysts for exhaust gas purification, wherein the catalyst at the exhaust gas inlet contains only Pd, the catalyst at the exhaust gas outlet contains Rh and Pt, or Rh, Pt and Pd, and the volume ratio of the former catalyst and the latter catalyst is 1:8 to 3:1.
The three-way catalyst disclosed in Japanese Patent Publication No. 38892/1991, however, was developed with the aim of allowing the catalyst to have an improved oxygen storability and have an activity in the vicinity of stoichiometric air-fuel ratio, and no performance at a fuel-lean side or at cold start was considered in the development. Further, in the above catalyst, the alumina layer has a large thickness of 20-40 mm and hinders gas diffusion into the catalyst layer containing Rh (Rh is most effective for the reduction and removal of NO.sub.x). Thus, the catalyst is not complete as a three-way catalyst.
Each of the catalysts each supported on an electrically heated catalyst (EHC), disclosed in Japanese Utility Model Application Kokai (Laid-open) No. 67609/1988, Japanese Patent Application Kohyo No. 500911/1991 and Japanese Patent Application Kokai (Laid-open) No. 72953/1991 is a conventional three-way catalyst and has a catalyst component (e.g. noble metal) supported on a refractive metal oxide (e.g. Al.sub.2 O.sub.3). Neither composition nor structure preferable for use with an electrically heated catalyst (EHC) is disclosed in any of the above documents.
The catalyst disclosed in Japanese Patent Application Kokai (Laid-open) No. 56247/1990 has no activity at a fuel-lean side where hydrocarbons can be oxidized and converted at a high efficiency. Thus, the catalyst has an insufficient activity for hydrocarbons.
The catalyst for exhaust gas purification disclosed in Japanese Patent Application Kokai (Laid-open) No. 84635/1988 is prepared by (1) dipping, in a Pd compound solution, the gas inlet portion of a carrier whose wall surface is coated with alumina or the like, by a desired distance, followed by drying and firing, (2) dipping, in a Pt compound solution, the gas outlet portion of the carrier by a desired distance, followed by drying and firing, and (3) dipping the entire catalyst layer in a Rh compound solution to impregnate the entire catalyst layer with Rh, followed by drying and firing. In this catalyst, since Rh and Pt or Pd (they form an alloy easily) are supported without being separated from each other, they form an alloy easily. Thus, the catalyst has low heat resistance.
Also in the catalytic converter disclosed in Japanese Patent Application Kokai (Laid-open) No. 101813/1991, no measure is taken for the prevention of alloy formation between Rh and Pt or Pd. Further, the catalytic converter was developed with the aim of improving the hydrocarbon conversion at a fuel-rich side and is not intended to improve the hydrocarbon conversion at a fuel-lean side in the presence of secondary air. Furthermore, the catalytic converter is not suitable as a heater type catalyst.
The present invention has been made in view of the above situation. The present invention is intended to provide (1) a catalyst for exhaust gas purification which can remove hydrocarbons efficiently at a fuel-lean side and which has an appropriate composition and structure also when viewed as a catalyst supported on the EHC, and (2) a method for exhaust gas purification which can convert unburnt hydrocarbons which are present in a large amount in the exhaust gas from a gasoline engine of an automobile at the engine start, at a high efficiency by using the above catalyst and introducing secondary air in front of the catalyst.